U.S. patent number 4,501,153 [Application Number 06/440,211] was granted by the patent office on 1985-02-26 for test machine for determining concrete strength.
Invention is credited to Ferenc Mehes, Andras Mozes.
United States Patent |
4,501,153 |
Mehes , et al. |
February 26, 1985 |
Test machine for determining concrete strength
Abstract
A test machine for determining the strength of concrete which is
suitable to perform quality tests of structures or structural
elements within the plant or at the site. The machine has a loading
unit provided with a force measuring means. A breaking cup is
embedded in the concrete to be tested when it is wet, an extractor
head being fixed to the breaking cup, serving for extraction
thereof from the solidified concrete, and between the force
measuring means and the extractor head there is inserted a joint
ending in a releasable connecting piece.
Inventors: |
Mehes; Ferenc (Budapest,
HU), Mozes; Andras (Budapest, HU) |
Family
ID: |
10949575 |
Appl.
No.: |
06/440,211 |
Filed: |
October 4, 1982 |
PCT
Filed: |
February 23, 1982 |
PCT No.: |
PCT/HU82/00006 |
371
Date: |
October 04, 1982 |
102(e)
Date: |
October 04, 1982 |
PCT
Pub. No.: |
WO82/02949 |
PCT
Pub. Date: |
September 02, 1982 |
Foreign Application Priority Data
Current U.S.
Class: |
73/803 |
Current CPC
Class: |
G01N
3/08 (20130101) |
Current International
Class: |
G01N
3/08 (20060101); G01N 003/08 (); G01N 033/38 () |
Field of
Search: |
;73/803,864.51,827,834,845 ;264/31 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ruehl; Charles A.
Attorney, Agent or Firm: Young & Thompson
Claims
What we claim is:
1. A test machine for determining the strength of concrete,
comprising a loading unit for loading the concrete until
destruction by applying tensile stress to a portion of the
concrete, a support for supporting the loading unit against a
surface of the concrete to be tested, a breaking cup adapted to be
embedded in wet concrete which when hardened is to be tested, said
breaking cup being hollow and having an open inner end which is the
end embedded deepest in the concrete, the side walls of said cup
converging toward said open inner end, and means connecting said
loading unit with said cup to draw said cup out of the concrete
after the concrete is set, thereby to fracture the concrete along a
plane extending across said open inner end of the breaking cup.
2. A test machine as claimed in claim 1, in which said side walls
of the breaking cup are a figure of rotation of an imaginary line
about an axis parallel to the direction in which the breaking cup
is drawn out of the concrete.
3. A test machine as claimed in claim 2, in which said figure of
rotation is a truncated cone.
4. A test machine as claimed in claim 3, in which said inner end is
circular.
5. A test machine as claimed in claim 1, in which said breaking cup
has an outwardly extending flange about its outer end opposite said
inner end, for attaching the breaking cup to the loading unit.
6. A test machine as claimed in claim 5, in which said means
connecting the breaking cup to the loading unit comprises an
extractor head to which said flange is releasably connected by
screws.
7. A test machine as claimed in claim 1, in which said open inner
end of the breaking cup has an inner surface which is a shallow
cylinder.
8. A test machine as claimed in claim 7, and a flexible ring
disposed in said shallow cylinder.
9. A test machine as claimed in claim 1, said means connecting said
breaking cup to said loading unit including a ball and socket joint
on the breaking cup.
Description
The invention relates to a test machine for determining concrete
strength, especially for performing quality tests of concrete
structures or structural elements within the plant or at the site.
The machine is provided with a loading unit loading the concrete
until destruction. The loading unit applies tension on some part of
the concrete and it is provided with a device for determining the
crumbling strength causing destruction e.g. with a force measuring
means. The force measuring means is set on a support arranged on
the surface of the concrete to be tested.
In the field of concrete and reinforced concrete building, when
either applying prefabricated structures or erecting monolithic
buildings, there is needed the possibility to determine technical
parameters of the solidified concrete, first of all the mechanical
properties thereof. From the earliest days of building with
concrete it has been customary to prepare particular test pieces,
so-called test-cubes or more rarely test pieces of special form,
which are loaded until destruction and from the crumbling strength
a conclusion can be drawn in respect of the concrete strength.
These tests provide, however, only informatory results and full
safety conclusions can not be drawn therefrom as to the mechanical
properties of the completed structure. The first and most important
reason for this resides in that, as a matter of course, the pouring
circumstances of the test pieces differ from those of the concrete
of the actual structure. Besides, solidifying of the raw concrete
may also take place under substantially different conditions.
Experience has shown that the strength parameters measured on test
pieces give an unrealistically good result. Thus, efforts have been
made for a number of years to perform also mechanical tests based
on fracturing samples taken from the actual structures and on
evaluation of the results of these fractures.
In the course of this latter procedure, samples e.g. test cubes are
cut out from the finished structure at predetermined locations,
then these are loaded until fracture. Such a method can, however,
in many cases not be used firstly on account of the damage to the
structure and secondly because there can be some special reasons
due to which the whole of the structure must not be broken down nor
the load thereon shifted by the stress accompanying the cutting
work. This was the motive leading to a solution wherein punches of
different forms are embedded in the concrete material before it
solidifies and from the force needed to extract them, a conclusion
may be drawn as to the mechanical properties.
The experiences with this test method were not unambiguously
advantageous since during extraction of the punch, the fractured
cross section will be accidental i.e. it will have a different form
and size in each case. Thus, in spite of carefully measuring the
force, the measured value is not reliable enough due to the
inaccurately known cross section. One of such measuring methods is
described in U.S. Pat. No. 4,103,540.
More reliable tests than the above-mentioned "punch extraction" are
those wherein from the values of the pore space and of the
water-cement coefficient a conclusion is drawn to the concrete
strength. One of these methods is described in the GDR Pat. No.
102,043. On more or less similar considerations are based also
those accelerated methods for determining strength described in
patents Nos. CD Pat. No. 2,607,919, U.S. Pat. No. 3,974,679 and Fr
Pat. No. 2,323,149. All these solutions have the common deficiency
that due to uncertain basic data, the conclusions to be drawn from
the test will be vague.
A further advance in concrete testing was achieved by procedures in
which the sample is cast in the concrete element to be tested. Such
methods are to be found in the patents Nos., DE Pat. Nos.
1,917,242, 1,917,730 and U.S. 3,176,053. According to these methods
so-called trial cores are cast in and then removed from the
concrete to be tested and thereafter they are loaded until fracture
on a testing machine.
As most expedient is found the variation of the latter method
according to which telescopic double sleeves are embedded in the
concrete element. Also with these more developed procedures the
uncertainty of the fractured cross section is more or less
prevalent and this results in the test results being hardly to be
compared to each other.
In the field in question, the most modern testing methods are
considered to be those using samples cut out from the solidified
concrete element and, from measuring the ultimate tensile load a
conclusion is drawn to the mechanical properties. Typical examples
of such methods are described in U.S. Pat. No. 3,861,201 and in the
essentially identical Austrian Pat. No. 320,313. This method is
suitable for determining strength properties of concretes and in
general subsequently solidified materials, wherein solids of
special shapes are embedded in the material and then extracted
after solidifying of the material.
This method contains several good ideas. However, it has the
drawback that the test result does not represent directly the
ultimate tensile strength but a fictitious crumbling strength which
is influenced also by a number of different parameters such as
compression strength, bending-tensile strength and even the
shearing strength of the concrete. Thus, it is very difficult to
compare the qualities of different concretes.
On a more or less similar consideration is based also the test
method serving to determine strength characteristics of materials
manufactured by casting and subsequent solidifying described in the
U.S. Pat. No. 3,541,845. The reliability of this method is less
than that of the former because the extracted test pieces are
loaded simultaneously by shearing stress and by tensile stress. The
stresses may, due to different local circumstances, develop and in
given case be dominant with a high degree of accidentality.
There have been similar experiments with apparatus serving solely
for testing concrete and reinforced structures as described in U.S.
Pat. No. 3,595,072. Its main drawback consists in that one can not
state a simple and strict correlation between the test results and
the actual so-called cube strength of the concrete.
The same is true of the disclosure in French Pat. No. 2,313,677. By
this latter method only the bending strength is to be stated and
also in this case the local faults of casting and of other
circumstances causing a wide variation of the test results and
casting doubt on the comparability thereof, play an important
role.
The invention aims at developing a test machine on the one hand
suitable to perform quality tests both in plants and at the site
and on the other hand retaining the advantages of the method
according to which the test piece is extracted from the concrete
element by simultaneously eliminating the deficiences originating,
in the case of the known solutions, from the uncertain cross
section of the fracture.
Within said field, the invention provides a solution giving direct
information about the strength of the built-in structure, enabling
tests to be performed at low costs in a quick and simple manner,
and performing tests with a minimum variance, with a high degree of
test safety and reliability.
The inventive idea is based on the recognition that direct
information relating to the built-in concrete element or structure
and simultaneously a test result in unique correlation to the cube
strength, can be achieved when the test machine by its construction
ensures that under the effect of the tensile load, the fracture
will result always at the same place, along a determined cross
section. Thereby not only can the high degree of variance
accompanying the known test machines be eliminated but at the same
time a simple device for quality control is achieved which can be
operated without special qualification.
In accordance with these objects, a test machine is to determine
the strength of concrete, especially to perform quality tests of
concrete structures or structural elements within the plant or at
the site, comprising a loading unit for loading the concrete until
destruction, the loading unit applying tensile stress on some part
of the concrete. The machine is provided with a device for
determining the crumbling strength causing destruction e.g. with a
force measuring means, and the force measuring means is set on a
support arranged on the surface of the concrete to be tested. In
the concrete to be tested, when it is wet, a breaking cup is
embedded, an extractor head being fixed to the breaking cup serving
for extraction thereof from the already solidified concrete and
fixed to the breaking cup by a force transmitting connection. A
joint ending in a preferably threaded connecting piece is inserted
between the force measuring means and the extractor head.
The invention will be described below in greater detail on the
basis of the accompanying drawings: In the drawings FIG. 1 is a
schematic longitudinal section through a possible embodiment of the
test machine,
FIG. 2 shows the longitudinal section of another embodiment,
FIG. 3 is the longitudinal section of a third embodiment,
FIG. 4 shows a possible embodiment of the supporting device used
instead of the supporting frame,
FIG. 5 shows the breaking cup, and
FIG. 6 shows an enlarged fragment of FIG. 5.
In FIG. 1 there is shown in schematic longitudinal section, a
breaking cup 6 embedded within the concrete 7, having a
frusto-conical mantle 6a with a cross section reducing in size from
the surface 7a of the concrete 7 inwardly of the concrete 7. To the
mantle 6a there is connected a collar 6b made preferably as a unit
therewith, being fixed rigidly yet in a releasable manner to an
extractor head 5 by screws 8 which are preferably parallel to the
longitudinal axis 6c of the breaking cup 6.
The extractor head 5 consists, according to FIG. 1, of a disc 5a
and a shaft 5b preferably formed as one piece. Along the periphery
of the disc 5a and of the collar 6b there are provided bores to
coincide with each other, in which the screws 8 can be set. Between
the extractor head 5 and the force measuring means known per se and
being not the subject matter of the present invention, there is
inserted a joint 4 ending in a connecting piece 2 and serving for
transmitting the load from the force measuring means 1 to the
extractor head 5.
The joint 4 may be connected to the extractor head 5 in a
releasable manner e.g. via threaded connection or bayonet locking.
The connecting piece 2 can preferably be turned in relation to the
joint 4, and the force measuring means 1 is provided with a housing
1a to receive the connecting piece 2.
To support the force measuring means 1 and to stabilize its
position in relation to the structure of the concrete 7, a
supporting frame 3 is provided. In the case of the embodiment shown
in FIG. 1 the supporting frame 3 consists of a base ring 3a set on
the surface 7a of the concrete 7, of a supporting disc 3b to set
the force measuring means 1 thereon, and of legs 3c connecting 3a
and 3b with each other.
Instead of legs 3c there can be provided a single "skirt" having a
mantle of e.g. truncated pyramid or truncated cone form. In either
case there is of course a need to provide in the supporting disc 3b
a central hole 3d to let the joint 4 pass through.
In FIG. 2 there is shown an extractor head 5 in longitudinal
section, wherein the disc 5a is provided with a liner 5d in the
form of a concave socket. The liner 5d forms with the ball 5a a
ball-and-socket joint, and enables a small relative turning between
the disc 5a and the shaft 5b.
Due to the special construction, the concrete cake 7b inside the
breaking cup 6 is extracted always by tensile load only. Thus, the
breaking cup 6 is never subject to an eccentric load causing
sticking and thereby disturbing the test result.
In FIG. 3 an embodiment is shown wherein the extracting force is
transmitted from the force measuring means 1 to the extractor head
5 via a screw-thread. In this case, the extractor head 5 is
provided with a threaded bore 5e engaging a threaded bolt 1b
forming part of the force measuring means 1.
In FIG. 4 the most simple form of the supporting frame 3 is shown
as a supporting ring, having a central hole 3d. Preferably, the
supporting ring is provided with a shoulder 3f suitable to set the
force measuring means 1 thereon.
In the course of the test, the breaking cup 6 with the downwardly
narrowing cross section is arranged in the concrete structure or
concrete element to be tested, at a predetermined place, when the
raw concrete is not yet solidified. It is advisable now to snap
some kind of a cap 10 (as shown in FIG. 5a) upon the collar 6b, in
order to prevent the raw concrete from penetrating into the bores
of the collar 6b.
The cap further serves to facilitate setting the upper face of the
collar 6b of the breaking cup 6 parallel to the surface 7a of the
concrete 7. It may also be of advantage to lubricate the outer side
of the mantle 6a of the breaking cup 6 with some anti-adhesion
material e.g. with grease or separating oil in order to prevent the
concrete from bonding there, as otherwise this would eventually
influence the test result.
After the concrete has solidified, at a predetermined time the cap
10 will be taken off the breaking cup 6, then by fitting the
extractor head 5 thereto and by inserting and fastening the screws
8 a force transmitting connection is provided through joint 4 and
connecting piece 2 with the force measuring means 1. By operating
the force measuring means 1, the extraction force is slowly and
progressively increased until the concrete cake 7b is torn off from
the surface of the concrete 7.
In FIG. 6 it can be seen that the end flange 6d of the breaking cup
6 on its side facing the concrete cake 7b, ends in a cylindrical
mantle 6e the generatrix of which is parallel to the longitudinal
axis of the breaking cup 6. By this construction it is ensured that
the concrete cake 7b shall be torn off along the flange 6d. In
certain cases the breaking circumstances may further be improved
when in the cylinder mantle 6e a flexible ring 9 made of e.g.
plastics or metal is inserted.
Experiments show that the quotient of the force value read off at
the moment of breaking, against the fractured cross section, gives
a strength value which is unambiguously characteristic of the
concrete as tested. This is due first of all to the fact that the
concrete cake 7b formed as a truncated cone will be torn off always
along a cylindrical mantle having a generatrix parallel to the
direction of the tensile force and perpendicular to the
longitudinal axis.
The measuring accuracy of the test machine according to the
invention fulfils even the highest production requirements. It is
suitable to replace the conventional cube breaking procedure. It
offers the possibility to perform measuring at several points on
the structure, enabling thereby better quality control and, through
the safety test, makes it possible to realize structures while
saving cement. The breaking cups 6 may remain in the concrete 7
since they are preferably made of stainless steel. Thus, during the
lifetime of the structure measuring can be performed also at a
later date and thereby it is possible to determine ageing
properties of the concrete.
* * * * *